The present invention generally relates to a high voltage generating apparatus for an X-ray tube and more particularly to an inverter type high voltage generator for use with an X-ray tube which can provide, with high accuracy, an X-ray tube voltage commensurate with a set voltage.
In the past, an X-ray tube high voltage generator has been used wherein a power supply voltage fed from a commercial power source is applied to a voltage adjustment transformer which adjusts its output voltage or the input voltage of the following transformer by changing the position of its sliding brush connected to the secondary winding thereof or changing output taps connected to the secondary winding, an output voltage of the transformer is transformed to a high voltage by means of a high voltage transformer, and the high voltage is rectified with a rectifier and applied to an X-ray tube.
Recently, a power control technique using power semiconductors has made drastic progress and has been applied to development of an inverter type high voltage generator for an X-ray tube. Thanks to use of semiconductors for power control, the inverter type high voltage generator has a response which is more rapid by far than that of the aforementioned generator using the voltage adjustment transformer. Accordingly, with the inverter type high voltage generator, it is possible to detect an X-ray tube voltage or a value corresponding to the X-ray tube voltage and to enable a feedback control for making an error or difference between a detected voltage and a set level equal to zero, thereby applying a relatively accurate high voltage to an X-ray tube.
FIG. 1 schematically shows a prior art inverter type, X-ray tube high voltage generator adapted for the feedback control. In the figure, there are illustrated a commercial power source 1, a rectifier 2 for converting an AC voltage into a DC voltage, a DC/DC converter 3 receiving the DC voltage outputted from the rectifier 2 and being responsive to a predetermined frequency f.sub.1 so as to be repetitiously on-off controlled to control its output DC voltage in accordance with a ratio between an on-time and an off-time (hereinafter referred to as a current conduction ratio), and an inverter 4 comprised of switching elements 4a to 4d. Simultaneous turn-on of the switching elements 4a and 4d and simultaneous turn-on of the switching elements 4b and 4c are effected repetitiously and alternately in response to a predetermined frequency f.sub.2 to apply to a primary winding 5a of a high voltage transformer 5 an AC voltage at the frequency f.sub.2. There are also seen in FIG. 1 a rectifier 6 for converting an AC voltage induced in a secondary winding 5b of the transformer 5, a capacitor 7 for smoothing an output DC voltage of the rectifier 6 to provide a smoothed DC voltage, i.e., X-ray tube voltage v.sub.x applied to an X-ray tube 8, an X-ray tube voltage detector 9 for detecting the X-ray tube voltage and producing a detection signal v.sub.x ' corresponding to the X-ray tube voltage, an error amplifier 10 for producing a signal a corresponding to an error between the detection signal v.sub.x ' and an X-ray tube voltage set level V.sub.set, and a DC/DC converter controller 11 responsive to the signal a to produce a signal b which controls the current conduction ratio for the DC/DC converter 3.
The error amplifier 10 is typically constructed as shown in FIG. 2, having resistors 21 and 22 of the same resistance R.sub.1, an operational amplifier 23, a resistor 24 of a resistance R.sub.2, and a capacitor 25 of a capacitance C.
In operation, when starting X-ray radiation, a DC voltage regulated to a predetermined level by the DC/DC converter 3 is converted by the inverter 4 into an AC voltage. This AC voltage is boosted by the high voltage transformer 5, rectified and smoothed by the rectifier 6 and capacitor 7, and applied to the X-ray tube 8. The tube voltage is detected by the X-ray tube voltage applied to the X-ray tube 8 detector 9, and a detection signal v.sub.x ' is inputted to the error amplifier 10 to change the level of signal a in accordance with an error from the X-ray tube voltage set level V.sub.set. In response to a resulting signal a, the DC/DC converter controller 11 produces a signal b which controls the current conduction ratio for the DC/DC converter 3. For example, when the detection signal v.sub.x ' is smaller than the X-ray tube voltage set level V.sub.set, the current conduction ratio for the DC/DC converter 3 is increased to raise the X-ray tube voltage v.sub.x. Conversely, when the detection signal v.sub.x ' exceeds the X-ray tube voltage set level V.sub.set, the current conduction ratio for the DC/DC converter 3 is minimized to decrease the X-ray tube voltage v.sub.x. In this manner, the X-ray tube voltage v.sub.x is controlled to make the error between the X-ray tube voltage v.sub.x and the X-ray tube voltage set level V.sub.set equal to zero.
Incidentally, the X-ray tube voltage v.sub.x takes a waveform which typically is rippled at a frequency of 2f.sub.2 as shown in FIG. 3. The ripple is caused by the operation of the inverter 4. In the inverter 4, the switching elements 4a and 4d in one set or the switching elements 4b and 4c in the other set should be turned on simultaneously and simultaneous turn-on of the switching elements 4a and 4c or the switching elements 4b and 4d should be prevented by providing a ceasing period T.sub.d to avoid short-circuiting of the output of the DC/DC converter 3. During the ceasing period T.sub.d, no power is transmitted from the DC/DC converter 3 to a succeeding load and consequently, the X-ray tube 8 is powered only by a discharging current from the capacitor 7, resulting in a decrease in the X-ray tube voltage v.sub.x. In addition, since a wiring inductance and a leakage inductance and a stray capacitance of the high voltage transformer 5 are coexistent with the inverter 4, high voltage transformer 5 and rectifier 6 and liable to cause load current to oscillate. For these reasons, the pulsation at the frequency 2f.sub.2 is caused irrespective of the stabilization of the DC voltage in the DC/DC converter 3 and is very difficult to reduce by controlling the DC/DC converter. Therefore, the pulsation at the frequency 2f.sub.2 must be separated from the feedback control.
In the error amplifier 10 shown in FIG. 2, the input/output relation is given by ##EQU1## where S is a parameter of Laplace transform. Thus, the output signal a delays by CR.sub.2 in responding to the input signal and for CR.sub.2 &gt;2f.sub.2 /1, the output signal a will not respond to the 2f.sub.2 frequency pulsation, thereby making it possible to provide stable control.
In this manner, the error amplifier can be adjusted so as not to respond to the 2f.sub.2 frequency pulsation by selecting CR.sub.2. But, this measure is equivalent to smoothing the 2f.sub.2 frequency pulsation and hence a smoothed value of the 2f.sub.2 frequency pulsation is considered to be added to a feedback level. In other words, as shown in FIG. 3, when the X-ray tube voltage has a maximum value V.sub.p, a level V.sub.m of the X-ray tube voltage which is averaged in respect of the pulsation is fed back.
The magnitude of the 2f.sub.2 frequency pulsation depends on the magnitude of the load. For larger X-ray tube currents, the pulsation is aggravated while for smaller X-ray tube currents, the pulsation is suppressed. The maximum value of the X-ray tube voltage v.sub.x is normally defined as an X-ray tube voltage. Accordingly, even when the 2f.sub.2 frequency pulsation for a small X-ray tube current and that for a large X-ray tube current are averaged to provide the same X-ray tube voltage level V.sub.m as shown in FIG. 4, the X-ray tube voltage is so controlled as to have a level V.sub.p1 for the small X-ray tube current and a level V.sub.p2 for the large X-ray tube current.
As described above, since the feedback control is effected through the error amplifier shown in FIG. 2 such that different X-ray tube voltages are to be equal to each other when different magnitudes of different pulsating waves are averaged to the same level, an error is caused between the actual X-ray tube voltage and the X-ray voltage set level.